Calorimeter types

The Calvet calorimeters have their roots in the work of Tian [26] and the later modifications by Calvet [7]. Presently this calorimeter type is commercially available from Setaram and the models C80 and BT215 are particularly well adapted for safety studies. It is a differential calorimeter that may be operated isothermally or in the scanning mode as a DSC in the temperature range from room temperature to 300°C for the C80 and -196 to 275°C for the BT215. They show a high... 

The course of the thermal power change can be determined by using different calorimeters. The choice of the heat effect determination method does not depend on the calorimeter type. In a calorimeter with a vacuum jacket, the thermokinetics can be determined successfully by means of the dynamic method. In calorimeters whose inertia is very small (conduction calorimeters), use of the flux method is not always suitable. Better results in the reconstruction of thermokinetics are obtained from the use of methods where it is assumed that the calorimeter is an inertial object of first or higher order. 

Applications Involving Liquid—Caseous Transition Although this calorimeter type is only of historical interest nowadays, one interesting application should, however, be mentioned Williams (1963) described a deformation calorimeter suitable for use with different liquids. 

For each of these control modes, different calorimeter types with different construction principles and different measuring principles exist (see Chapters 1 and 5). For reasons of clarity, we restrict ourselves in what follows to these three operation modes and collect the different calorimeters in three groups without further detailed classification characteristics. Numerous calorimeters belong to these groups, and it is not possible to present all of them in particular, we are not able to list all calorimeters that are commercially available but only typical examples to explain the respective characteristic properties. 

For measurement of the heat exchanged between the sample in the calorimeter vessel and the surroundings, different methods exist, which have already been explained in Part One of this book. We will not repeat the fundamentals in this chapter unless the details are of essential importance for the respective calorimeter type. 

This heat flow calorimeter type was originally developed by Suurkuusk and Wadso (1982) for direct and continuous monitoring of the very small heat effects... 

This type of calorimeter is nomrally enclosed in a themiostatted-jacket having a constant temperature T(s). and the calorimeter (vessel) temperature T(c) changes tln-ough the energy released as the process under study proceeds. The themial conductivity of the intemiediate space must be as small as possible. Most combustion calorimeters fall into this group. 

A liquid serves as the calorimetric medium in which the reaction vessel is placed and facilitates the transfer of energy from the reaction. The liquid is part of the calorimeter (vessel) proper. The vessel may be isolated from the jacket (isoperibole or adiabatic), or may be in good themial contact (lieat-flow type) depending upon the principle of operation used in the calorimeter design. 

The selection of the operating principle and the design of the calorimeter depends upon the nature of the process to be studied and on the experimental procedures required. Flowever, the type of calorimeter necessary to study a particular process is not unique and can depend upon subjective factors such as teclmical restrictions, resources, traditions of the laboratory and the inclinations of the researcher. 

Combustion or bomb calorimetry is used primary to derive enthalpy of fonuation values and measurements are usually made at 298.15 K. Bomb calorimeters can be subdivided into tluee types (1) static, where the bomb or entire calorimeter (together with the bomb) remains motionless during the experiment (2) rotating-... 

Recent developments m calorimetry have focused primarily on the calorimetry of biochemical systems, with the study of complex systems such as micelles, protems and lipids using microcalorimeters. Over the last 20 years microcalorimeters of various types including flow, titration, dilution, perfiision calorimeters and calorimeters used for the study of the dissolution of gases, liquids and solids have been developed. A more recent development is pressure-controlled scamiing calorimetry [26] where the thennal effects resulting from varying the pressure on a system either step-wise or continuously is studied. 

Thermochemistry is concerned with the study of thermal effects associated with phase changes, formation of chemical compouncls or solutions, and chemical reactions in general. The amount of heat (Q) liberated (or absorbed) is usually measured either in a batch-type bomb calorimeter at fixed volume or in a steady-flow calorimeter at constant pressure. Under these operating conditions, Q= Q, = AU (net change in the internal energy of the system) for the bomb calorimeter, while Q Qp = AH (net change in the enthalpy of the system) for the flow calorimeter. For a pure substance. 

In the combustion reaction as carried out in the calorimeter of Figure 7-2, the volume of the system is kept constant and pressure may change because the reaction chamber is sealed. In the laboratory experiments you have conducted, you kept the pressure constant by leaving the system open to the surroundings. In such an experiment, the volume may change. There is a small difference between these two types of measurements. The difference arises from the energy used when a system expands against the pressure of the atmosphere. In a constant volume calorimeter, there is no such expansion hence, this contribution to the reaction heat is not present. Experiments show that this difference is usually small. However, the symbol AH represents the heat effect that accompanies a chemical reaction carried out at constant pressure—the condition we usually have when the reaction occurs in an open beaker. 

The ABC cereal company is developing a new type of breakfast cereal to compete with a rival product that they call Brand X. You are asked to compare the energy content of the two cereals to see if the new ABC product is lower in calories so you burn 1.00-g samples of the cereals in oxygen in a calorimeter with a heat capacity of 600. J-(°C). When the Brand X cereal sample burned, the temperature rose from 300.2 K to 309.0 K. When the ABC cereal sample burned, the temperature rose from 299.0 K to 307.5 K. (a) What is the heat output of each sample (b) One serving of each cereal is 30.0 g. How would you label the packages of the two cereals to indicate the fuel value per 30.0-g serving in joules in nutritional Calories (kilocalories) ... 

Figure 6-17 illustrates a constant-volume calorimeter of a type that is often used to measure q for combustion reactions. A sample of the substance to be burned is placed inside the sealed calorimeter in the presence of excess oxygen gas. When the sample bums, energy flows from the chemicals to the calorimeter. As in a constant-pressure calorimeter, the calorimeter is well insulated from its surroundings, so all the heat released by the chemicals is absorbed by the calorimeter. The temperature change of the calorimeter, with the calorimeter s heat capacity, gives the amount of heat released in the reaction. 

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

All calorimeters are composed of an inner vessel (the calorimeter vessel, A in Fig. 1), in which the thermal phenomenon under study is produced, and of a surrounding medium (shields, thermostat, etc., B in Fig. 1). Depending upon the intensity of the heat exchange between the inner vessel and its surroundings, three main types of calorimeters may be distinguished theoretically as indicated in Fig. 1. 

When heat is liberated or absorbed in the calorimeter vessel, a thermal flux is established in the heat conductor and heat flows until the thermal equilibrium of the calorimetric system is restored. The heat capacity of the surrounding medium (heat sink) is supposed to be infinitely large and its temperature is not modified by the amount of heat flowing in or out. The quantity of heat flowing along the heat conductor is evaluated, as a function of time, from the intensity of a physical modification produced in the conductor by the heat flux. Usually, the temperature difference 0 between the ends of the conductor is measured. Since heat is transferred by conduction along the heat conductor, calorimeters of this type are often also named conduction calorimeters (20a). 

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